A large portion of international transport is performed by using containers. Containers are transportation units having standardized dimensions and used to contain the transported goods during the transportation. Typically containers exist in three sizes, having lengths of 20 feet, 40 feet or 45 feet. The width of the containers is typically 2.5 meters.
The containers are handled in container terminals typically located at harbours or inland. At the container terminals, the containers are handled by using particular cranes, including a Rail Mounted Gantry Crane (RMG-crane), and a Rubber Tyred Gantry Crane (RTG-crane), for example. A particular type of the RMG-crane is a ship-to-shore crane that is used to unload containers from the ship to the pier and load the containers from the pier to the ship.
The cranes are equipped with an attachment part for attaching to the container. A typical attachment part is a spreader that has variable dimensions to allow handling of different sizes of containers. Spreaders known as tandem and twinlift spreader can attach to two or more containers at a time.
Typical handling of containers includes operations such as stacking and picking up of containers. For example 5 containers can be stacked on top of each other. The stacking requires great precision from the person driving the crane, since the corners of the stacked containers have to be aligned with a precision of at least 5 cm. If the stacking is not performed with sufficient precision the whole stack may fall.
During handling of the containers, the spreader experiences vibrations and shocks from various sources, for example when attaching the spreader to a container for picking up the container and when lowering a container attached to the spreader to the ground, ship, trailer or chassis, or on top of another container. The vibrations and shocks make it difficult to handle the containers with sufficient precision. The vibrations and shocks are especially present in container handling, when a container carried by the spreader is released, e.g. when containers are stacked on top of each other or to the ground. On the other hand, damping of the vibrations and shocks may induce a delay to the operations performed in the containers. Increased delay causes a reduction to the efficiency of the container handling.
Depending on the weight of the container attached to the spreader, the spreader may be bent by the weight of the container. When the container is detached from the spreader, the spreader bends back again to its original shape. The bending makes it difficult to measure dimensions, since a spreader supports a heavy container at a lower height, when the spreader is bent.
Typically a spreader is attached to a crane via head block, using similar twist lock mechanism that is used in spreaders to attach to containers. The head block is connected to the ropes used for hoisting the spreader. The twist lock mechanism typically has some clearance to allow the spreader and the head block to move with respect to each other. However, when the position of the spreader is determined by the position of the head block, especially in automated operation of the spreader, the clearance introduces inaccuracy to the positioning of the spreader.
Crane operations are increasingly automated to provide faster handling of containers. Typically automated operations are followed by the driver via display. The driver can be located in the cabin of the crane or in a remote location from the crane. This means that operations performed on the containers are highly dependent on the correct operation of the automation equipment and the information conveyed to the driver by the system. Malfunctions or maintenance of the automation equipment leads to downtime of the crane during which the container traffic is on hold.
Automated crane operations typically involve positioning of the spreader for example by means of cameras and lasers. Positioning of the spreader is also described in the Finnish national patent application No. 20115757, which is incorporated herein by reference in its entirety.
Sensors may be installed to cranes to facilitate their automation. Sensors are typically installed to high altitudes, for example to trolley structures 20 meters above the ground. With such distances even the smallest of errors in the positioning of the sensors, e.g. angle, can have a very dramatic effect on the accuracy of operations performed at the ground level. Installation and calibration of such sensors is often a cumbersome and a labor intensive task. Additionally, the calibration as a process is complex and requires a specialized person to perform it.